Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 230201 AAA

Area Forecast Discussion...UPDATED
National Weather Service Chicago/Romeoville, IL
901 PM CDT THU SEP 22 2016

859 PM CDT

The going forecast message remains in good shape with a mild
evening for late September seeing festering scattered showers and
occasionally some lightning in eastern Iowa into northwest
and north central Illinois slowly inching eastward.

A diffuse frontal zone remains stationary over the area with a
northern stream cold front moving southward across northern
Wisconsin and northern Lake Michigan. Convergence within deep
moisture (1.64 in. PWAT at DVN) across the broad frontal an narrow but pronounced warm advection zone
aloft...continues to support the scattered showers. MUCAPE on the
00Z DVN sounding was nearly 900 J/kg, so some occasional isolated
thunder and brief heavy downpours will remain possible as the
minimal forcing slides eastward. Somewhat uncertain on what
shower coverage will be during Friday morning, and its possible
that is quite isolated.

Otherwise we remain in the warm sector for most of the night.
While there are some high dew points immediately along the
boundary, clouds and scattered showers should prevent much of a
fog threat. The one exception may be near the lake front in the
early morning as winds turn onshore behind the northern stream
cool front. This could steer in some fog, though that is
conditional on some fog actually developing over the water and it
may actually be an atmosphere more favored for stratus. Still
looking like the northern CWA should be mainly cloudy through a
good part of the day on Friday.



215 PM CDT

Through Friday...

The area of showers across northwest Iowa will continue to drift
east along a lingering stationary front. The airmass ahead of
this area is not super conductive for this activity to remain
quite as intact as it currently is, and there is very little
lightning activity with it as well. Think that some showers or
sprinkles will enter Northern Illinois as the afternoon continues.
Coverage should diminish with eastward extent.

The front will remain across the same area tonight. Several
shortwaves in wnw flow coupled with a strengthening low level jet
will trigger additional showers and thunderstorms tonight across
Central Iowa and into southwest Wisconsin, and some of this looks
to extend into our area overnight into Friday. Moisture and
instability across most of the area remains benign, but this area
could form into a mesoscale convective system overnight. It could
certainly turn SE through our area tomorrow, but the better
instability to our southwest tends to suggest thunder would remain
both north closer to the elevated warm front, and also west closer
to the LLJ, which could then support the case for highest pops
along and north of i-88 and more western areas. This may be
another situation of monitor observations as the convection
develops as model handling of these near term details sometimes
remains poor until storms form.

In addition...a secondary front across WI will move south tonight
as high pressure builds to the north. Low level winds will turn
northeast with this front. Stratus and fog will accompany the front
as it moves through northern Illinois tomorrow morning. There will
likely be a day with a decent temperature gradient across the area
with far northeast IL in the lower 70s to solidly into the mid 80s
into central IL. With a lingering frontal boundary across the area
and weak waves traversing the flow, there could be additional
showers, but instability should be weak with some cloud cover and
with an upper ridge building in, therefore will keep some pops
north of I-80.



306 PM CDT

Friday night through Thursday...

Primary forecast concern in the extended is with temperature and
precip trends through the weekend. A period of cooler and
potentially showery weather looks to arrive next week.

A highly amplified upper pattern will be in place across the
nation by the weekend, characterized by a large trough over the
western CONUS with a closed low over the central Rockies, and a
sharp downstream ridge across the Mississippi River Valley. At the
surface, low pressure will be in place across the high Plains,
with a nearly stationary frontal boundary extending into central
IL/IN. Surface high pressure across Ontario and the Great Lakes
region will supply somewhat cool easterly low level flow to the
forecast area. While some potential will exist for isolated light
and disorganized light precip near and north of the surface front
Friday night, height rises and generally subsident conditions are
anticipated across the western Lakes, which is expected to
strongly limit the extent of any light rain. The strongest large
scale forcing, and moisture return in southerly flow will be
across the central and northern Plains through Saturday, ahead of
the upper low which lifts northeast through the period, and a
surface cold front which trails the associated surface low.
Locally, precip potential should increase late Saturday night as
height falls begin to spread across the upper Midwest, and south-
southwesterly low level flow increases moisture transport into the
region. Rain chances continue as the trailing occluding cold front
slowly moves east across the cwa Sunday and Sunday night. With the
front south of the area Saturday, and east surface winds across
the area, a large gradient in temperatures are expected. Guidance
depicts mid-80`s across the south/southwestern cwa Saturday
afternoon while those east winds limit readings to the 70`s across
northeast IL. Similar trends are anticipated Sunday, though with
lower 80`s spreading a bit farther north as the warm front lifts
north ahead of occluding surface cold front approaching from the

Medium-range guidance slows the eastward progression of the upper
trough across the region early next week, with indications of
further amplification of the pattern across the Great Lakes by
mid-week. 12Z runs of GFS/ECMWF and GEM all indicate relatively
deep upper trough across the area, which suggests the potential
for a period of showery and relatively cool (low-mid 70`s) through
the remainder of the extended forecast period.



For the 00Z TAFs...

An area of showers, with an embedded thunderstorm, continues to
shift eastward across north central Illinois. However, AMDAR
soundings out of KMDW indicate the presence of some really dry air
around 750 MB, so I would anticipate these showers to weaken as
they shift towards the Chicago area terminals over the next 1-2
hours. As a result, no more than a light sprinkle is anticipated
with this activity. Scattered showers will be possible over the
area tonight into Friday, but there is no big signal to put a
mention in the TAFs at this time.

The main concern will be the increasing possibility of some lower
CIGs and northeasterly winds for Friday following a cold frontal
passage. It appears this front will move south over most terminals
late tonight with northeasterly winds expected for Friday. The
main change for the 00z TAFS was to include some lower end MVFR
CIGS at the main terminals by daybreak Friday. IFR CIGS are
possible, but confidence is too low to include in the forecast at
this time. It appears that MVFR CIGS could be around for much of
the morning, and possibly most of the day as well, as cooler air
continues to move in off the lake.



218 PM CDT

A frontal boundary is draped across Lake Michigan this afternoon
with fresh southerly flow south of the boundary and northeast winds
north. High pressure centered over Manitoba this afternoon will
slowly shift east into Ontario tomorrow and into the eastern Great
Lakes Saturday. As this occurs, the front will shift south across
Lake Michigan with a fresh to strong northeast breeze overspreading
most of the lake by early Friday, and then veering to east/southeast
Saturday. A slow moving low will traverse from the northern Plains
into the western Great Lakes this weekend into early next week with
winds continuing to veer to the south/southwest by Monday.






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